Continuous surface treatment for coils made of aluminum alloys sheets

ABSTRACT

The invention relates to a continuous coil surface treatment process suitable for both a coil of a 5XXX aluminum alloy sheet and of a 6XXX aluminum alloy sheet. The process comprises the successive steps of etching the surface with a fluorine-free acidic solution; rinsing with deionized water; applying a conversion solution comprising titanium and zirconium, and drying. The invention surface treatment method is environmentally friendly and enables the production of the treated surfaces in an efficient and reliable manner for both 5XXX and 6XXX alloys. It is particularly adapted for the production of aluminum alloy sheets for the automotive industry.

FIELD OF THE INVENTION

The invention relates to the field of continuous surface treatment ofsheets and strips made of an aluminum alloy, and parts stamped fromthese sheets, and particularly 5xxx or 6xxx type alloy according to theAluminum Association, intended for manufacturing bodywork parts forautomobile vehicles

BACKGROUND

Aluminum is increasingly used in automobile construction to reduce theweight of vehicles and therefore fuel consumption and releases ofpollutants and greenhouse effect gases. Sheets are used particularly formanufacturing bodywork skin parts, such as hoods and doors andstructural parts. This type of application requires a set of sometimescontradicting mechanical strength, corrosion resistance and formabilityproperties; with an acceptable cost for mass production.

For automobile parts, surface preparation adapted to assemblyoperations, and particularly adhesive bonding and welding, may benecessary. These pre-treatments take time and are expensive, inparticular because the bath are corrosive and require special precautionof health and environment. Recent treatments are chromium-free forenvironmental reasons and use elements such as organophosphorous, silaneand derivatives, titanium and/or zirconium. For example, such treatmentsare described in patents and applications U.S. Pat. No. 5,514,211. U.S.Pat. Nos. 5,879,437, 6,167,609, US2013/284049, US2011/041957,US2016/319440.

The patent FR2856079 describes an alternative treatment simplified byusing an atmospheric plasma.

Patent U.S. Pat. No. 5,868,872 discloses a chromium-free no-rinseprocess in particular, to applications in the food packaging industry.This patent does not disclose an etching step prior to treatment butmerely acidic or alkaline cleaners.

Patent U.S. Pat. No. 6,562,148 discloses a method for the pretreatmentof work pieces having a surface made of aluminium or aluminium alloyscomprising rinsing with an aqueous, acidic solution containing a mineralacid, rinsing with water, bringing into contact with an aqueous, acidicsolution which is chromium-free and polymer-free and contains Ti and Zras complex fluorides in a weight ratio of Ti:Zr of 2:1 to 12.

Patent application EP 2 537 674 A1 discloses an aluminum or aluminumalloy material having a surface treatment coating film on a surface of asubstrate formed by an aluminum or aluminum alloy, the surface treatmentcoating film being capable of maintaining hydrophilicity, high corrosionresistance, antibacterial properties and deodorizing properties for along period of time.

Surface preparation is usually carried out with a continuous coilsurface treatment line that enables the surface treatment of coils ofsheets.

The surface treatment line may comprise continuous solution heattreating and quenching or may be a line dedicated to surface treatment.Several surface treatment steps are applied to progressively modify thesurface properties in order to comply with an application of theproducts in the automotive sector, by optimizing the weight of achemical layer. A first surface treatment step is usually a degreasingstep of the sheet, which is used to remove oil residues on the sheet,present after the last rolling steps. Following the rolling processes,the surface of the aluminum includes a “disturbed area” also named“disturbed layer” or “disturbed surface layer”, and etching is used in asecond surface treatment step to remove the oxide layers and thedisturbed area, ensuring a uniform surface, which is more favorable todeposit a product such as a Ti/Zr conversion layer and for furthercorrosion/bonding durability properties. A third surface treatment stepis a conversion coating, the objective of the conversion being toprecisely deposit a layer weight (for example comprising of Ti and/orZr) which promotes adhesion and corrosion resistance. It has beengenerally admitted that the use of fluorine in the second etchingtreatment is necessary to eliminate the “disturbed area” and obtain asurface favorable to deposit the conversion layer. The use of fluorinecontaining bath requires however special precaution of health andenvironment. Another requirement is that the surface treatment linesdedicated to automotive applications have to able to treat alternativelycoils of 5XXX and 6XXX alloys sheets. It is thus economicallyadvantageous to keep the same treatment for different types of alloys.

It is a purpose of this invention to develop improved surface treatmentmethods that are more environmentally friendly to produce the treatedsurfaces in an efficient and reliable manner providing treated surfacesfor both 5XXX and 6XXX alloys.

SUMMARY OF THE INVENTION

An object of the invention is a continuous coil surface treatmentprocess suitable for both a coil of a 5XXX aluminum alloy sheet and of a6XXX aluminum alloy sheet having a surface comprising the successivesteps of:

a) optionally cleaning the surface of the aluminum alloy sheet;

b) etching the surface of the optionally cleaned aluminum alloy sheetwith a fluorine-free acidic solution;

c) rinsing the surface of the etched aluminum alloy sheet with deionizedwater;

d) applying to the surface of the etched aluminum alloy sheet aconversion solution comprising titanium and zirconium, with a zirconiumto titanium weight ratio of from about 3.0 to about 5.0 preferably from3.2 to 4.0;

e) optionally rinsing the surface of the converted aluminum alloy sheetwith deionized water; and,

f) drying the surface of the aluminum alloy sheet.

DESCRIPTION OF THE FIGURES

FIG. 1a is a SEM micrograph of a 6016 surface etched with a fluorinecontaining etching solution and FIG. 1b is a SEM micrograph of a 6016surface etched with a fluorine-free etching solution.

DESCRIPTION OF THE INVENTION

All aluminium alloys referred to in the following are designated usingthe rules and designations defined by the Aluminium Association inRegistration Record Series that it publishes regularly, unless mentionedotherwise.

Unless otherwise specified, the definitions of standard EN 12258 apply.

The method according to the invention is a continuous coil surfacetreatment process suitable for both a coil of a 5XXX alloy sheet and ofa 6XXX alloy sheet. One significant advantage of the invention is thatthere is no need to change or modify the treatments when switching theline from a 5XXX alloy to a 6XXX alloy.

The method is suitable for the treatment of a coil made of 5XXX alloysheet and for the treatment of a coil made of a 6XXX alloy sheet.Preferably the coil is a coil of a sheet made from an aluminum alloyselected from the group consisting of AA5754, AA5182, AA6451, AA6605,AA6005, AA6005A, AA6016, AA6116, AA6022, AA6013, AA6056, AA6156, AA6111and AA6014.

In a first step, the sheet surface may optionally be cleaned. Thecleaning may be used to remove residual oil left by the rolling process.Optionally, the cleaning may be carried out by hot water spraying and/orby using organic solvents and/or by using surfactants and/or detergentssuch as alkaline detergents. For some continuous treatment lines, thesheets may have been sufficiently degreased by the previous thermaltreatments and the cleaning step may not be needed. Also in someinstances cleaning and etching may be carried out simultaneously tosimplify the process. For productivity and cost reasons, it is usuallypreferable to avoid having a cleaning step.

In a next step, the surface of the optionally cleaned aluminum alloysheet is etched with a fluorine-free acidic solution. The presentinventors have found that by combining a fluorine-free acidic solutionwith a conversion bath comprising Ti and Zr in specific quantities it ispossible to treat 5XXX and 6XXX sheets with a same bath and obtainsatisfactory conversion. Preferably etching with a fluorine-free acidicsolution is carried out chemically and not electrochemically.

Preferably, the fluorine-free acidic solution comprises sulfuric acid,nitric acid, phosphoric acid, or mixtures thereof. Advantageously, thefluorine-free acidic solution comprises at least 80% sulfuric acid andphosphoric acid, or mixtures thereof. In an embodiment the fluorine-freeacidic solution does not contain nitric acid. It is advantageous if themixture of acids, if used, comprises at least 80% sulfuric acid.Advantageously, the concentration of sulfuric acid of the fluorine-freeacidic solution is from about 2 g/l to about 60 g/l, preferably from 15g/l to 50 g/l. Optionally, the fluorine-free acidic solution can includeone or more additives (e.g., surfactants and/or detergents) and/or oneor more accelerators. The surfactant and/or detergent additives can beincluded in the fluorine-free acidic solution at concentrations rangingfrom about 0.05 wt. % to 3 wt. %. Preferably, the surfactant and/ordetergent additives can be included in the fluorine-free acidic solutionat a concentration ranging from about 0.1 wt. % to 2.5 wt. %, from about0.2 wt. % to 2 wt. %, from about 0.3 wt. % to 1.5 wt. % or from about0.4 wt. % to 1.3 wt. %. A suitable accelerator that can be included inthe fluorine-free acidic solution includes ferric sulfate. Theaccelerators can be included in the fluorine-free acidic solution atconcentrations ranging from about 0.005 wt. % to 0.4 wt. %. For example,the accelerators can be included in the fluorine-free acidic solution ata concentration ranging from about 0.01 wt. % to 0.3 wt. %, from about0.03 wt. % to 0.2 wt. %.

The fluorine-free acidic solution can be heated to a temperature of fromabout 55° C. to about 85° C. The fluorine-free acidic solution can becontrolled within the outlined temperature and concentration rangesusing, for example, heat exchangers and metering pumps and overflows andsuitably replaced or replenished.

The fluorine-free acidic solution can be applied by spraying thesolution onto the sheet or by immersing the sheet in a bath. Preferably,the fluorine-free acidic solution is applied by immersion. Optionally,the fluorine-free acidic solution can be circulated to ensure a freshsolution is continuously exposed to the sheet surfaces. In anadvantageous embodiment, the fluorine-free acidic solution iscontinuously treated with an exchange resin in order to maintain alimited quantity of Al and Mg ions in the solution. Preferably, thefluorine-free acidic solution comprises less than about 3 g/l ofaluminum ions and magnesium ions. Advantageously, the fluorine-freeacidic solution can contain less than 2 g/l of aluminum and magnesiumions. The contact time for the acid etching step can be from about 5seconds to about 30 seconds or preferably from about 10 to about 20seconds.

Advantageously, the etching removal is from 0.01 to 0.2 g/m², preferablyfrom 0.01 to 0.1 g/m² for 6XXX alloys and from 0.1 to 0.2 g/m² for 5XXXalloys. With the fluorine-free acidic solution of the invention, theetching removal is substantially lower than the etching removal knownfrom prior art fluorine containing acid solution. This is advantageousto limit Al and Mg ions in the solution. Also, the etching of theinvention provides an advantageous smoother surface than the prior artetching, with in particular less or even no pitting.

After etching it is needed to rinse the surface of the etched aluminumalloy sheet with deionized water. Preferably, the deionized water inthis step has a conductivity of less than or equal to 50 μS/cm. Therinsing step is preferably performed at a temperature ranging from about37° C. to about 70° C. Advantageously, the rinsing step can be performedat a temperature of from about 40° C. to about 65° C. and preferablyfrom about 45° C. to about 60° C. The rinse can be a progressivecascading system. Preferably, sprays are used for the rinsing step.

The following step is applying to the surface of the etched aluminumalloy sheet a conversion solution comprising titanium and zirconium,with a zirconium to titanium weight ratio of from about 3.0 to about 5.0preferably from 3.2 to 4.0. The present inventors have found that bycombining etching with the fluorine-free acidic solution and conversionwith the conversion solution according to the invention, it is possibleto obtain a Zr/i weight ratio on the treated sheet surface of from about0.8 to about 1.3 for 5XXX and 6XXX alloys. This ratio is important forthe adhesive bonding properties of the sheets. A benefit of theinvention that it is not needed to modify the conversion solutioncomposition in order to treat both 5XXX and 6XXX alloys. The conversionsolution can be applied at a temperature of from about 35° C. to about65° C. Preferably, the conversion solution is applied at a differenttemperature for 5XXX and 6XXX, typically at a temperature of from about40° C. to about 50° C. for 5XXX alloys and of from about 45° C. to 55°C. for 6XXX alloys. The conversion solution may be applied for a contacttime of from about 5 seconds to about 20 seconds. Preferably, theconversion solution is applied at during a different contact time for5XXX and 6XXX, preferably with a contact time of from about 5 seconds toabout 9 seconds for 5XXX alloys, typically with a contact time of about7 seconds for 5XXX alloys and preferably with a contact time of fromabout 8 seconds to about 12 seconds for 6XXX alloys, typically with acontact time of about 10 seconds for 6XXX alloys.

Preferably, the conversion solution comprises titanium and zirconium,with a titanium content of from about 20 to about 200 mg/i andpreferably from 60 to 140 mg/i. Advantageously the conversion solutioncomprises hexafluotitanic acid, hexafluozirconic acid, hydrofluoric acidand optionally ammonium hydrogendifluoride. The additional fluoride(under the form of hydrofluoric acid and optionally ammoniumhydrogendifluoride) is advantageously added to complex the Al generatedduring treatment with the conversion solution. The pH of the conversionsolution, is advantageously adjusted upward to a pH from about 2.5 toabout 4.5, preferably from 3.5 to 4 by the addition of an alkali such asammonium hydroxide or ammonium hydrogendifluoride. In an advantageousembodiment, the conversion solution is continuously treated with anexchange resin in order to maintain a limited quantity of Al and Mg ionsin the solution. Preferably, the conversion solution comprises less thanabout 80 ppm of aluminum ions and magnesium ions.

The conversion solution can be applied by spraying the solution onto thesheet or by immersing the sheet in a bath. Preferably the conversionsolution is applied by spraying the aluminum alloy sheet with theconversion solution, with preferably at least 5 spraying ramps. Applyingthe inventive described conversion solution by roll-coating would not beoptimal because the Ti/Zr ratio on the sheet would be the same as theTi/Zr ratio in the conversion solution.

The following step is optionally rinsing the surface of the convertedaluminum alloy sheet with deionized water.

The final step is drying the surface of the aluminum alloy sheet. Thedrying step removes any water from the surface of the sheet. The dryingstep can be performed using an air dryer or an infrared dryer. Thedrying step can be performed for a time period of up to five minutes.The drying step may also be used for a metallurgical purpose, inparticular for 6xxx alloys if needed, so that the coiling temperature isadvantageously of from 50° C. to 120° C. or preferably from 60° C. to100° C.

Example

The surface of aluminum alloy sheets was treated on a continuous lineaccording to the methods described herein. The sheets included 5182aluminum alloy sheets and 6016 aluminum alloy sheets. The sheets werecleaned and etched by spray with a sulfo fluorhydric bath (H₂SO₄ 8 g/lHF 0.4 g/l, 50° C.) for trials 1 and 2 or with a sulfuric acid bath(H₂SO₄ 43 g/l 70° C.) for trials 3 to 6. SEM micrograph of the surfaceobtained for 6016 alloy etched with the sulfo fluorhydric bath (trial 2)and micrograph of the surface obtained for 6016 alloy etched with thesulfuric acid bath (trial 6) are presented in FIGS. 1a and 1brespectively. The etching of the invention provides an advantageoussmoother surface than the prior art etching, with much less pitting.Following etching the sheets were rinsed with deinoized water andtreated by spray with Ti/Zr conversion surface treatment with a pH of3.8 comprising hexafluotitanic acid, hexafluozirconic acid, hydrofluoricacid and ammonium hydrogendifluoride. The contact time, concentrationand temperature are shown in Table 1.

The quantity of Zr and Ti deposited on the surface was measured by XRFon three positions across the width for 6 samples. The average value ofthe measurement is provided in Table 1.

TABLE 1 Zr/Ti Ti Zr/Ti weight concentration weight ratio on inconversion ratio in the Conversion solution conversion Contact Ti Zrsheet bath (mg/l) solution Alloy Temperature time (mg/m2) (mg/m2)surface 1 A  40 2.8 5182 45° C.  6 5.2 5.5 1.0 2 6016 50° C.  8 5.4 5.10.9 3 B 100 2.2 5182 45° C.  7 5.4 5.1 0.9 4 6016 50° C. 10 6.1 3.4 0.65 C 100 3.5 5182 45° C.  7 5.1 6.9 1.3 6 6016 50° C. 10 5.4 4.6 0.9

Conversion coating with bath C is according to the invention. With themethod of the invention it is possible to obtain a Zr/Ti ratio between0.8 and 1.3 with a single bath for both 5XXX and 6XXX alloys, and obtainan advantageous smoother surface.

1. A continuous coil surface treatment process suitable for both a coilof a 5XXX aluminum alloy sheet and of a 6XXX aluminum alloy sheet havinga surface, said process successively comprising: a) optionally cleaningthe surface of the aluminum alloy sheet; b) etching the surface of theoptionally cleaned aluminum alloy sheet with a fluorine-free acidicsolution; c) rinsing the surface of the etched aluminum alloy sheet withdeionized water; d) applying to the surface of the etched aluminum alloysheet a conversion solution comprising titanium and zirconium, with azirconium to titanium weight ratio of from about 3.0 to about 5.0optionally from 3.2 to 4.0; e) optionally rinsing the surface of theconverted aluminum alloy sheet with deionized water; and, f) drying thesurface of the aluminum alloy sheet.
 2. The process of claim 1, whereinthe conversion solution comprises titanium and zirconium, with atitanium content of from about 20 to about 200 mg/l and optionally offrom 60 to 140 mg/l.
 3. The process of claim 1 wherein the conversionsolution comprises hexafluotitanic acid, hexafluozirconic acid,hydrofluoric acid and optionally ammonium hydrogendifluoride.
 4. Theprocess of claim 1, wherein the fluorine-free acidic solution comprisessulfuric acid, nitric acid, phosphoric acid, or mixtures thereof.
 5. Theprocess of claim 1, wherein the concentration of sulfuric acid of thefluorine-free acidic solution is from about 2 g/l to about 60 g/l,optionally from 15 g/l to 50 g/l.
 6. The process of claim 1, wherein thealuminum alloy sheet is made from an aluminum alloy selected from thegroup consisting of AA5754, AA5182, AA6451, AA6005, AA6605, AA6005A,AA6016, AA6116, AA6022, AA6013, AA6056, AA6156, AA6111 and AA6014. 7.The process of claim 1, wherein the applying in (d) is performed byimmersing the aluminum alloy sheet in the conversion solution.
 8. Theprocess of claim 1, wherein the applying in (d) is performed by sprayingthe aluminum alloy sheet with the conversion solution.
 9. The process ofclaim 1, wherein the etching in (b) is performed by spraying thealuminum alloy sheet with the fluorine-free acidic solution.
 10. Theprocess of claim 1, wherein the fluorine-free acidic solution in (b)comprises less than about 2 g/l of aluminum ions and magnesium ions. 11.The process of claim 1, wherein the fluorine-free acidic solution,comprises one or more surfactant additives or accelerators and is usedat a temperature of from about 55° C. to about 85° C. for a period ofabout 5 to about 30 seconds.